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See detailElaboration d’un modèle d’écobilan de l’exploitation agricole irriguée. Cas du périmètre du Tadla – Maroc
Krim, Lhassan ULg

Doctoral thesis (2011)

In Morocco, the agro environmental problem is mainly translated by a degradation of the waters quality and a soils salinization under irrigation. In the, studied zone of the present work, the lands ... [more ▼]

In Morocco, the agro environmental problem is mainly translated by a degradation of the waters quality and a soils salinization under irrigation. In the, studied zone of the present work, the lands irrigation started since several decades by using waters of sometimes very degraded mineral quality, on one hand, and the agricultural intensification essentially marked by the application of excessive nitrogenous fertilizers doses, on the other hand, engendered a strong nitric pollution of groundwater table and a spectacular salinization of soils and waters. In order to contribute to the environmental assessment of the irrigated agriculture in Morocco, an Ecobalance model baptized "EcoTadla" (TEB) was elaborated and validated in the Tadla perimeter through the adaptation of the "EcoFerme" model (MEF) developed by the Fluid Mechanics and Environment Unity of the Gembloux Agro-Bio Tech – University of Liege (Belgium). The general objective is to establish an Ecobalance model allowing to quantify and to analyse the materials fluxes exchanged between the farm and its environment and consequently to improve and\or to correct the agricultural practices aiming at a better agro environment preservation within an eco-advice initiative framework. The analysis of the inadequacies of MEF to make an operational model in the studied context allowed to bring the adaptations to be brought as regards the definition of the basic hypotheses to hold in the new model construction, the quantification of the parameters to be used as inputs of the model and the modifications to be introduced into the modelling of the fluxes equations. The TEB model subdivides the farm into three compartments the “Cultivated soil”, the “Cultures” and the “Animals”. The farm (studied system) exchanges materials fluxes with its environment subdivided, in his turn, into four compartments: The “Network waters”, the “Under ground” the "Atmosphere" and the "Third parties". The Ecobalance calculation period is of twelve months equivalent to an agricultural campaign and elements, taken into account at present, in the fluxes calculation are: the nitrogen (N), the phosphor (P), the potassium (K), the calcium (Ca), the magnesium (Mg), the chlorine (Cl), the sodium (Na) and the water (H20). Considering the specificities of the agro environmental problem in the studied context, the TEB global model was conceived as a general program which calculates the Ecobalance for the various taken elements and which offers the possibility to the user to focus the study on three sub models: The "Water" model (WEB), the "Nitrogen" model (NEB) and the "Salts" model (SEB). These three models allow to quantify and to analyse the materials fluxes respectively for "Water", "Nitrogen" and "Salts" on the farm or perimeter scale during the period calculation. The TEB model was tested, verified and validated on an optimised network of representative farms. In order to characterize better the studied domain and clarify more the main parameters and the inputs required by the model, this network was the object of a farming follow-up in the form of inquiries, local measures and analyses in the laboratory of samples of water and soil during three agricultural campaigns (2002-2003, 2003-2004 and 2004-2005). WEB sub model quantify a total of 15 fluxes between these compartments which are two types: "major fluxes" and "minor fluxes". The lysimétrique follow-up led in two contrasted farms allowed to conclude that this model is reliable with an error risk varying from 17 to 26 %. The irrigation water use efficiency remains globally weak with an average of 40 % exceeding 50 % only in 25 % of the farmers of the follow-up network while the irrigation valuation water rate in the production is 56 % on average. Therefore, it’s necessary to award more eco-advice efforts to improve the efficiency and rationalize the irrigation water use. The analysis of the model sensibility demonstrated that the varying fact of this rate of 5 % and 10 % leads a variation respectively of 17 % and 36 % on the final result represented mainly by the infiltration rate which is the vector of the nitrogenous pollution and the salts washing. Ultimately, the Web model is a tool that can be used by the manager to improve eco advice on recovery and rational use of water in agricultural irrigation. Similarly, the quantification of the indicators presented would help the manager make the right choice of crops for better utilization of irrigation water The NEB sub model allows quantifying 17 fluxes between the farm and its environment. The nitrogen potential lixiviation flux, main indicator of the pollution risk the groundwater, is on average 77,8 kg N ha-1 on the follow-up network scale with a maximum of 105 kg N ha-1 and a minimum of 35 kg N ha-1. The lysimetric follow-up curried out shows that 14 % and 13 % of the residual mineral nitrogen was respectively washed during the 2003-2004 campaign and the 2004 2005campaign. So it’s necessary to make more eco advice efforts in terms of nitrate fertilizers use. The analysis of sensibility of the model showed that the rate mineralization variation of 5 % and 10 % engendered a variation of the “nitrogen potential lixiviation” flux respectively of 10 and 19 %. The NEB sub model is very useful for the manager can learn about the potentially leachable inorganic nitrogen and the nitrogen fertilizer made unnecessarily by comparing the doses recommended. The SEB model quantifies 8 main fluxes mostly salts conveyed by irrigation waters of and washing salts. On 15 farms of the Ecobalance follow-up network, this “potentially washable salts” flux is about 4210 kg ha-1 on average with a variation coefficient of 52 %. The lysimetric follow-up realized showed that 18 % and 12 % of salts were respectively washed during the 2003-2004 campaign and the 2004-2005 campaign. Consequently, it’s required to think about rehabilitation measures within an interdisciplinary framework aiming at the durability preservation of the production systems. The analysis of sensibility of the model showed that the electric conductivity rate variation of 5 % and 10 % engendered a variation of the «washing salts" flux respectively of 8 % and 16 %. The analysis of SEB model results in relation with the NEB model in 10 farms studied showed that, given the quality of water and soil, the reasoning of the nitrogen should be reflected according to the timetable and efficiency of irrigation-dependent process of leaching of salts, but also according to the vegetative stage of the plant for better utilization of soil mineral nitrogen. Ultimately, a policy of eco efficient and appropriate advice to the problem of environmental study area inevitably involves consideration of three elements "water", "nitrogen" and "salts ". The TEB program could be one of the fundamental tools of policy All these results impose to think to technical support strategies to set up follow-up devices of the ground, to strengthen the capacities of taking targeted measures to validate and verify the developed models and cheer the work in interdisciplinary by developing the collaboration such as integrated projects aiming at the sustainability of the agricultural production systems. By more clarifying the parameters used as inputs, these models would become more successful and more reliable [less ▲]

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